Use of mechanical vibration produced at an ultrasonic frequency to weld thermoplastics, and to emboss and form plastics is a well-established industrial process. The physical principles underlying this technology have important relations to the invention described herein and therefore merit brief review and discussion.
To obtain significant vibrational motion, most ultrasonic systems are operated at one of their frequencies of resonance. Both the ultrasonic generator and the ultrasonic horn are designed to resonate at the same frequency, in which case the vibration produced by the generator is communicated to the horn. Since the horn is tuned to the same frequency as the generator, the horn expands and contracts along its length in concert with the imposed motion of the vibration generator.
The motion produced at the free face of the horn is then reciprocal, or back and forth in a surface perpendicular to the surface of the horn, with an amplitude determined by the electrical voltage applied to the crystals of the vibration generator. It is known to condition the vibrations produced by the generator before the vibrations are communicated to the horn, including incorporating amplification devices and phase change devices into the sequence of elements so used.
One problem encountered in applying resonant ultrasonic vibration to continuous processes is the limitation on horn width. Although the horn executes motion principally of contraction and extension along its working surface, the wider the horn, e.g. the longer its working surface, typically the greater the variation in the amplitude of the vibrations along the length of the working surface. Accordingly, in general, use of ultrasonic energy to process workpieces in a continuous web has been limited to two types of processes.
The first type of process is one where a rotary ultrasonic horn disposed in a fixed location applies ultrasonic energy, against an underlying anvil, at essentially a line along a workpiece travelling, in a longitudinal direction along the workpiece, past the rotating horn. This first type of process is illustrated by U.S. Pat. 3,222,235 to Buchner.
The second type of process is one where one or more stationary flat planar surface or plunge-type, as they are commonly referred, ultrasonic horns extend across the width of the web and apply ultrasonic energy to all or selected parts of the web as the web passes between the stationary horns and corresponding underlying anvils, as illustrated by U.S. Pat. No. 3,939,033 to Grgach et al and U.S. Pat. No. 3,733,238 to Long et al, respectively. The plunge-type may imply its motion, however, the horn may, in fact, be stationary and the web moved to it.
In a first modification of the second type of process, U.S. Pat. No. 4,713,132 to Abel et al teaches mounting a series of flat planar surface horns for rotation about fixed axes disposed about a rotating anvil drum, and rotating the horns such that the horns rotate into and out of contact with the workpieces on the web.
In a second modification of the second type of process, U.S. Pat. No. 4,650,530 to Mahoney et al teaches folding the web of workpieces about the periphery of a rotating disc, and bringing ultrasonic horns, and corresponding anvils into alignment with the workpieces, as they rotate on the disc. The horns and anvils are of the flat planar surface arrangement. The horns and anvils rotate with the disc, and the ultrasonic system operates on the workpieces by bringing respective sets of horn and anvil against the respective workpiece at a locus in a free space between radially extending protrusions on the disc. Thus the disc which carries the workpieces does not participate in the application of ultrasonic energy, and the reciprocating motion of the ultrasonic horns operates in a direction coincident with the direction of extension of the axis of the rotating disc which carries the workpieces.
A second problem with flat planar surface ultrasonic horns is that, in order to obtain uniform application of the ultrasonic energy along the length of the horn, the spacing of the horn from the anvil must be aligned in two dimensions along the face of the horn such that the working surface of the horn is e.g. precisely parallel with the working surface of the anvil. Especially where there is e.g. any continuing motion of the horn or the anvil other than the ultrasonic vibrations, for example rotation of either or both of the horn or anvil, such precise alignment becomes a dynamic problem, requiring constant monitoring and repeated adjustment of the spacing between the horn and the anvil.
It is an object of this invention to provide an ultrasonic system including an ultrasonic horn and a cooperating anvil wherein one of the horn and anvil is mounted on a rotating web-carrying work drum, and the other of the horn and anvil is mounted for rotation with the work drum, and to extend over the work drum to apply ultrasonic energy to a workpiece and to withdraw from over the work drum during each rotation of the work drum.
It is another object to provide an ultrasonic system wherein the anvil is mounted on the work drum and the ultrasonic horn is mounted for rotation with the work drum, and for extending over the work drum and withdrawing from over the work drum during each rotation of the work drum.
It is still another object to provide an ultrasonic system wherein the horn is a rotary ultrasonic horn adapted to traverse across the width of the workpiece web, applying pressure and ultrasonic energy, thus to provide line application of ultrasonic energy to the workpiece, moving progressively across the workpiece on the web.
Still another object is to provide such a system including a plurality of sets of horns and anvils disposed about the circumference of the work drum, wherein the system can simultaneously process a related plurality of workpieces.
Yet another object is to provide an ultrasonic system wherein one or more ultrasonic horns of the plunge type are mounted on the rotating work drum, and a corresponding anvil is mounted for rotation with the work drum, and for extending over the work drum and withdrawing from over the work drum during each rotation of the work drum.
A further object is to provide an ultrasonic system wherein one or more ultrasonic horns of the plunge type are mounted on the rotating work drum and the respective anvil is a rotating anvil adapted to traverse across the width of the workpiece web, applying pressure against the respective horn, thus to provide line application of ultrasonic energy to the workpiece, moving progressively across the web.